1. Field of the Invention
The present invention relates to a Radio Frequency Integrated Circuit (RFIC) included in a receiver of a wireless telecommunications system. More particularly, the present invention relates to an apparatus and method for compensating for a direct voltage offset which is generated in an RFIC.
2. Description of the Related Art
In a conventional wireless telecommunications system, the double conversion or the Heterodyne receiver was mainly used. Recently, there is an interest in a direct conversion receiver, also known as a Zero-IF receiver, because it has an advantage in performing a direct conversion into a required frequency without an intermediate frequency generation process, thus simplifying the process as compared to a dual conversion receiver. That is, the dual conversion receiver performs a removal of adjacent channel interference and a gain control according to strength and weakness of received signals in an intermediate frequency unit.
However, since the direct conversion receiver performs the removal of adjacent channel interference and the gain control in a baseband, a receiver can be implemented by a filter having a low quality factor in the direct conversion receiver in comparison with the dual conversion receiver. However, an amplifier included in the direct conversion receiver is difficult to design due to Noise Factor, Linearity, Variable Gain Section, Gain Control Step, Accuracy and Direct Voltage offset (DC-offset). Among them, the Direct Voltage offset is recognized as the most serious problem.
In the meantime, since the frequency of the carrier frequency of the received signal and the frequency of the local oscillator are substantially identical due to the structure of the direct conversion receiver, it is not easy to remove the direct voltage offset generated in the inside of receiver in the baseband. Moreover, since the Radio Frequency Integrated Circuit (RFIC) itself is manufactured using a Complementary Metal Oxide Semiconductor (CMOS) process for integration and miniaturization, there can be a problem that the direct voltage offset generated in the CMOS element is very highly amplified in a section where gain is high even in case a direct voltage offset applied to the baseband amplifier does not exist. In order to compensate for such direct voltage offset, in the related art, a method of compensating for an offset by itself in the inside of an RFIC and a method of removing an offset with reference to a stored value after storing the offset compensation value into memory in association with a modem in the initial driving were proposed.
However, the former method for compensating for an offset has a problem in that it can be used only when there is sufficient time to remove direct voltage offset since much time is required to reach a steady-state in the alteration of gain. Particularly, in the Long Term Evolution (LTE) system in which a time for offset compensation is restricted to be less than 4 μs in the normal operation, it is impossible to compensate for the direct voltage offset by the former method of compensating for an offset. Accordingly, generally, the latter method of removing an offset is used so that the direct voltage offset compensation is applicable within 1 μs. However, the latter method has a problem in that offset can be changed by a low frequency noise generated due to the CMOS characteristic.